More aggressiveness in treatment of childhood malignancies has had an evident impact on survival and rate of cure but, it has also allowed us to discover long-term effects of these treatments, and second malignant tumors of them. Between 1970 and 1993, 472 cases of malignant tumors in childhood were diagnosed in our department. Six of them (1.27%) developed a second tumor (five malignant and one benign). Relationship between first and second tumors are: seven years old boy, cervical lymphosarcoma-thyroid carcinoma; eleven years old boy, osteogenic sarcoma-vesical carcinoma: two years and six months old boy, cerebellar astrocytoma-soft tissue osteogenic sarcoma; five years old girl. Wilm's tumor-scapular osteogenic chondroma; one year and a half old girl, abdominal neuroblastoma-granulocytic sarcoma (chloroma); twelve years old boy. Hodgkin's disease-acute myeloblastic leukemia. All of them were clearly related to concogenic effect of radiation or chemotherapy.